In cranial facial skeleton tissues, cartilage serves a number of critical functions including support, locomotion, and growth. This lattel activity is mediated through a specialized tissue, the endochondral growth plate. The life cycle of growth plate chondrocytes is shod and dependent upon local and systemic factors. These factors regulate all stages of chondrocyte activity including terminal differentiation and death. Recent studies indicate that the local oxygen tension may also serve to regulate growth plate chondrocyte activity. The purpose of the proposed investigation is to elucidate the mechanism by which oxygen tension regulates the maturation and terminal differentiation of growth plate chondrocytes. Attention is focused on two oxygen-sensitive proteins, the oxygen sensor prolyl hydroxylase (PHD) and the transcription factor, hypoxia inducible factor (HlF). Work in progress suggests that PHDs sense oxygen by modification of specific proline residues in HIF. HIF then transcriptionally activates downstream genes concerned with "regulating a wide range of activities including glycolysis, angiogenesis, maturation, and apoptosis. How PHD and HIF control ;hondrocyte function is unknown. There are three Specific Aims: (1) To measure the expression of PHD and HIF isoforms during :hondrocyte maturation in vitro and in vivo; to relate expression of each of the transcripts to selected stages of the chondrocyte life cycle. It is hypothesized that chondrocyte maturation is accompanied by a stage specific expression of both PHDs and HIF. (2) To selectively delete and overexpress HIF and PHD isoforms in chondrocytes; to determine how these factors regulate chondrocyte :lifferentiation and mineralization of the extracellular matrix. It is hypothesized that PHD and HIF are required for chondrocyte maturation in an avascular environment and regulate chondrocyte oxidative metabolism. (3) To examine the relationship between the HIF expression and the sensitivity of chondrocytes to apoptosis; to ascertain if HIF expression modulates the activity of proteins required for the induction of cell death. It is hypothesized that expression of HIF sensitizes chondrocytes to apoptosis.